1. Field of the Invention
The present invention pertains to liquid crystal on silicon (LCOS) displays, and more particularly to improved temperature and color temperature control and compensation method for the microdisplay systems.
2. Description of the Prior Art
Since microdislay systems, especially the liquid crystal on silicon (LCOS) Microdisplay frequently operate in the hot interior of a projection device, the microdisplay technology is still challenged by the need to effectively control the temperature and compensate for the color balancing under the circumstances of temperature increase such that the quality of display would not be impaired by uncontrolled high temperatures. The difficulties of color balancing are compounded because the display from each color element has its own individual temperature variations and each color element also has different temperature sensitivities. Meanwhile, it is imperative to control and proper compensate the color balancing operated under temperature variations because the color balance of a projection system is an important feature of its performance.
In a well-designed system, the color balance is determined by the respective power levels of the primary colors and by the spectral bandwidths of those colors. Various techniques have long been known in the art that can be used to achieve color balance in a projection display system where the intensities of the three colors can be modulated separately. In the application of such techniques to projection systems based on microdisplays and spatial light modulator, some problems arise. First, the microdisplays most often operate in the hot interior of a projection device. As will be further discussed below, all components within such devices have thermal sensitivities of some sort. The birefringence of the liquid crystal material within such a display normally becomes lower with elevated temperature and thus the electro-optical (EO) curve for such a device is highly temperature dependent. In a system using three separate microdisplays the situation often arises where each of the microdisplays operates at a different temperature than the others. When the unit is first turned on after having previously reached ambient temperature the microdisplays are all operating at lower than normal temperature. While the rise in temperature begins immediately it may take 30 minutes to reach a new, stable set of operating temperatures. The voltage transfer curve has been shown to vary with temperature. Additionally, the voltage-transfer curves for each color device at a given temperature differ because of the differences in the materials. A technical challenge is faced by the microdisplay system to provide a method of determining the temperature of the liquid crystal to develop and implement control methods that mitigate the effects of high or low temperature through temperature control or other compensation and that simultaneously maintain proper color balance.
There are several prior art approaches taken in attempt to solve the problems caused by temperature variations in a microdisplay system including disclosures made by 1) U.S. Pat. No. 6,304,243, Kondo, et al, “Light Valve Device” Oct. 16, 2001, column 28, line 62 through column 29, line 37, for a discussion of one approach to the implementation of cooling of a microdisplay; 2) U.S. Pat. No. 4,338,600, Leach, “Liquid Crystal Display System Having Temperature Compensation” Jul. 6, 1982, and 3) U.S. Pat. No. 4,460,247, Hilsum et al, “Temperature Compensated Liquid Crystal Displays”, Jul. 17, 1984. Another disclosure was reported by Kurogane et al to use an electro-optic mode that does not exhibit noticeable thermal variation in the linear region of interest. However, the availability of the materials employed and special manufacture processes and mode of operations would significantly restrict the usefulness of the proposed microdisplay systems. Another is the approach taken in U.S. Pat. No. RE 37056, Wortel, et al, where the inventors disclose a method to manufacture the cell in such a manner that the slopes of the electro-optic curves measured at different temperatures in the same liquid crystal device are quite close. A simple temperature measurement system is employed to provide information to a system that can adjust the column drive voltage and thus effect the compensation. However, this particular approach is of limited usefulness because the method requires a very specific approach to the design and manufacture of the cell.
In view of the current state of the art of microdisplay temperature control, there is an ever-increasing demand for new methods and system configurations that can effectively control the temperature and to compensate the performance variations caused by the temperature changes due to the temperature sensitivities of the microdisplay systems. There are several reasons for such increased demand. First, it is observed from operations of microdisplay systems that a liquid crystal experiences a rise in temperature from ambient over a period of 20 to 30 minutes after a system is turned on. This rise in temperature is attributable in part to a rise in ambient temperature within the product case due to heating of the air within by such items as the lamp and by other electronic components. A second major source of heating is the heat generated from the thermal characteristics of the silicon in the LCOS microdisplay itself. A third major source is heat caused by the illumination from the lamp falling on the microdisplay itself. The degree of temperature increase depends on the thermal design of the product and the environment in which it operates. A second reason for the increasing demand to control and compensate temperature effect for a microdisplay system is a observation that the system performance of a microdisplay is strongly temperature dependent. A first sensitivity of LCOS microdisplays is the reduction of the birefringence of the liquid crystal material with elevated temperature within such a display with thus the electro-optic (EO) curve for such a device is highly temperature dependent. One particular aspect of this temperature driven effect is that the dark state rises as temperature deviates from the design temperature and therefore the contrast of such a system suffers.
FIG. 1A shows the strong influence of the temperature changes on the electro-optic performance of a nematic liquid crystal cell constructed by using a 45° twisted nematic (45° TN) in normally black (NB) electro-optic mode. The cell is nominally 5.5 μm thick. The clearing temperature of the liquid crystal is not precisely known but is estimated to be 85° C. Four sample temperature curves determined by experiment are depicted. Thus the major effects of the temperature variations are clear upon inspection. First, the liquid crystal (LC) curve shifts to lower voltage as the temperature of the LC rises. Second, the intensity of the achievable dark state rises as temperature rises. The apparent magnitude of the dark state intensity appears to increase nonlinearly as temperature rises. Third, the location of the peak of the voltage curves shifts to lower voltages as the temperature rises. Fourth, the height of the peak of the voltage curve drops slightly as temperature rises. Finally, the voltage required to achieve the best dark state (whatever that is) does not appear to move significantly with changes in temperature.
Referring to the LC curves of FIGS. 1B and 1C disclosed in U.S. Pat. No. RE 37,056 for further understanding of the temperature dependence of the performance of a microdisplay system. FIG. 1B shows diagrammatically transmission/voltage characteristics of a display device according to the invention at different temperatures, while FIG. 1C shows similar characteristics for a conventional display device. The data as illustrated in FIGS. 1B and 1C are curves for normally white mode transmissive displays which are also representative of reflective mode normally white displays as well. As disclosed in the patent, FIG. 1B presents data that is better behaved than that of FIG. 1C. Implicit in the patent itself in describing the difficulty is the likelihood that the liquid crystal cell is being driven by an analog drive source, such as a Digital-to-Analog Converter (DAC). The DAC would have to be adjusted to a completely different slope and origin in configuring it to drive at different temperature in the case of FIG. 1C. The control and compensation of temperature variation for microdisplay system according to the disclosed techniques would become more cumbersome and inconvenient due to this adjustment requirement.
Thus from the above it is clear that temperature is an important factor in the performance of a liquid crystal device. It is also clear that knowledge of the temperature of a liquid crystal device can enable several commonly known control mechanisms in the electro-optical-mechanical design of a product using such devices. In order to control the microdisplay operational temperature, traditional measures includes the use of fan controlled by a thermostat for activating a fan to increase the air circulation of a microdisplay system. Alternatively the thermostat may be position to measure the heat at a set of heat sinks mounted to the back of the microdisplays. Additionally, the knowledge of several control mechanisms in the electro-optical-mechanical design embodied in different products using such mechanisms can be implemented to further exploit such knowledge to achieve optimal performance. However, as of now, the conventional technologies in microdisplay temperature control still have not fully take advantage of the availability of different control mechanisms to improve and enhance the temperature control and compensation for microdisplay systems operated under widely varying temperatures. Particularly, temperature compensations for adjusting color contrast in response to temperature variations to achieve improved color balancing become more important when the microdisplay systems are subject to greater degree of temperature variations.
Color balance in a system has two important aspects. The first is the range of colors that can be created in a system. This is referred to as the color gamut of the system. It is determined by the spectrum of the color used to create the primary colors of the system. This information is commonly presented as an x-y plot of the color coordinates of the three primaries; the most common system being the CIE 1931 color plots. Colors that can be created by these primaries will have color coordinates that fall within the triangle formed by the three primaries. The x-y coordinates of colors that fall outside the triangle cannot be represented by such colors. The primary colors themselves, in a three-panel projection system, are determined by the spectral characteristics of the lamp, by the various optical filters and the pass characteristics of the optical elements, and by the efficiency and spectral response characteristics of the light modulators. A CIE 1931 plot with indicates of regions associated with particular colors, from page 7 of Hazeltine Corporation Report No. 7128, “Colorimetry”, dated Jun. 10, 1952, which in turn cites D. B. Judd, “Color in Business, Science and Industry” John Wiley and Sons, 1952, is shown As FIG. 1D.
The second important aspect of color balance is the color temperature of the white point of the system. In its simplest form the white point of a system is determined by the color coordinates when all three channels are turned on to their maximum intended brightness. This can be measured reliably using instruments such as those used to measure the color coordinates of the primaries. The determination of color temperature requires assessment of the color coordinates against an overlay of the black body curve. A useful version of the curve, presented in FIG. 1F, that shows a chart in CIE 1931 format with the coordinated color temperature and black bodylines. FIG. 1E includes cross lines that indicate the positions of the coordinated color temperature. Coordinates along the line are psychologically considered to be approximately the same color temperature, although they are not exactly the same color.
The color coordinates of the white point of the system are determined not only by the color coordinates of the individual primaries, but by the relative power of the primaries. The relative power of the primaries is normally determined in large part during the design phase when a new projection device is made. It requires a comprehensive assessment of the filtering function of each component within a system, including the microdisplays. FIG. 1F is a sample spectral filtering arrangement showing a typical set of band-pass limits for each color with efficiency superimposed on the normalized lamp spectrum for a high-pressure mercury lamp. In FIG. 1F, the x-axis scale is in the unit of nanometer.
Given a set of performance characteristics, the color coordinates for each spectral channel can be predicted; although it is often preferable to measure the color coordinates experimentally to take into account component variance from the nominal specifications. Similarly, the white point can be predicted from measured data or calculated data, although a direct measurement is a more reliable method. Regardless of the origins of the data, it is clear that changes to the efficiency of the individual color channels will change the relative intensity of portion of the spectrum and therefore will change the color coordinates of the white color point, hence the color temperature of white.
As discussed above, the spectral band-pass limits are normally designed into the system early in its development. While changes can be made, this normally requires the replacement of a spectrally important component, such as a dichroic trim filter or the like. In some cases, dichroic filters are designed and then mounted to facilitate rapid modification of a design.
Furthermore, since the microdisplays are sensitive to variations from the design temperature. In the instances presented, the voltage required to reach maximum efficiency drops as temperature rises. Additionally, it is experimentally proven that the microdisplay for each color may be operating at different liquid crystal temperatures. It is also well known that the curve of voltage versus efficiency is normally different for each color, even in those instances where the liquid crystal cells are identical. This is because the longer wavelengths interact differently with a given cell configuration.
Managing a constant white point under such circumstances is challenging but can be accomplished if the ambient conditions are those predicted by the designers. However, there are always circumstances where the ambient cannot match the exact circumstances predicted. One example is that of a system that has just been turned on and is going through a warm-up period. A second likely circumstance is that the room temperature is hotter or colder than the nominal design temperature for the mechanical design of the system, resulting in the introduction of air into the system that differs from the design expectation to some degree.
For these reasons, there is still need and great challenge in the art of microdisplay such as a three-panel liquid crystal on silicon (LCOS) display to provide improved system architecture and methods of temperature control and color-balancing and compensation to improve the system performance under wide ranges of temperature variations such that the above-mentioned limitations and difficulties can be overcome.